In the art of applying coatings comprising magnetizable particles dispersed in a nonmagnetizable elastomeric binder, a dispersing agent is normally employed to disperse the magnetizable and any other fine particles, usually in amount of 10-50% by weight of the binder composition. Since the dispersing agent remains in the coating, it tends to have a plasticizing effect which weakens the binder. Consequently, the coating exhibits inferior wear resistance and eventually releases debris. Furthermore, the dispersing agent can contribute to the migration of low-molecular-weight materials within the coating and their exudation onto the recording surface.
Dispersing agents are likewise used in applying coatings comprising fine particles which are not magnetizable such as the conductive carbon black of controlled-wind backside coatings of many magnetic recording tapes. These and any other coatings comprising dispersions of fine particles in elastomeric binders involve the same problems.
Some binders and some dispersing agents which are highly effective for the magnetizable particles of a given recording medium may be less effective for the non-magnetizable particles of its magnetizable and backside coatings, thus possibly requiring an inventory of more than one binder and more than one dispersing agent.
The above-mentioned debris problem may be minimized if the dispersing agent has one or more functional groups which will react with crosslinking agent for the binder plus one or more additional groups which will be adsorbed by the particles of the dispersions. See U.S. Pat. No. 3,460,984 (Bisschops et al.). British Pat. No. 1,132,957 (Graubart) employs a melamine-formaldehyde resin as a dispersing agent and it may also function as a crosslinking agent in a polyurethane binder of a magnetic recording medium.
A dispersing agent which crosslinks the binder tends to react with the binder in the coatable dispersion so that within a fairly short period of time, the dispersion no longer can be used. Hence, the dispersion must be made up in small batches shortly before use.
U.S. Pat. No. 3,490,945 (Slovinsky) discloses a polyurethane binder for magnetic recording media which itself is a dispersing agent. The polyurethane binder is a moisture-cured diisocyanate-terminated prepolymer containing surface-active tertiary or quaternary nitrogen. While this theoretically should have solved the aforementioned problems, it is believed that to date there has been no elastomer on the market which both acts as a high-quality binder and also effectively disperses inorganic particles. The polyurethane binder of the Slovinsky patent may not have become commercial because of the impracticability of formulating the binder under dry conditions and keeping the dispersion under a dry atmosphere, and the moisture-curing might cause some foaming.
U.S. Pat. No. 3,419,420 (Stahly) also discloses a recording tape binder which has a dispersing capability, but it too has apparently not become commercial.